In image display devices such as cathode ray tube (CRT) displays, liquid crystal displays (LCD), plasma displays (PDP) and electroluminescence displays (ELD), the outermost surfaces are generally provided with optical laminates for anti-reflection. Such optical laminates for anti-reflection minimize virtual image formation and reduce reflectance by diffusion and interference of light.
One type of known optical laminate for anti-reflection is an antiglare film having an irregularly shaped antiglare layer formed on the surface of a transparent base material. The antiglare film can diffuse external light and prevent reduction in visibility caused by external light reflection or virtual image formation, by the irregular shape of the surface.
An example of a known conventional antiglare film is one wherein a resin containing a filler such as silicon dioxide (silica) is coated on the surface of a transparent base material film to form an antiglare layer (see PTLs 1 and 2, for example).
Such antiglare films include types in which irregular shapes are formed on the surface of the antiglare layer by aggregation of particles such as cohesive silica, types in which an organic filler is added to the resin to form irregular shapes on the layer surface, and types in which a film having irregularities on the layer surface is laminated to transfer the irregular shapes.
All such types of conventional antiglare films are designed to produce a light diffusing and antiglare effect by the action of the surface form of the antiglare layer, and require larger and a greater number of irregular shapes to increase the antiglare property, but when the irregularities or their number are increased, this raises the haze value of the coating film, causing discoloration and concomitantly lowering the contrast.
As the opportunity to view displays with high image quality movies and the like have increased in homes as well, there is increased demand for blackness of black screens in dark rooms (hereunder referred to as “blackness in dark surroundings”).
The haze exhibited by surface irregularities is defined as the “surface haze” while the haze exhibited when smoothing has been performed using a resin that forms surface irregularities, or a resin with a difference in refractive index of at least 0.02 from such a resin, is defined as the “internal haze”, and these are measured according to JIS K 7136 (2000).
The haze value, or the ratio of the internal haze and total haze, is commonly used as a simple method for evaluating contrast. Specifically, it has been considered that an optical sheet with low contrast reduction can be produced by specifying the materials and controlling the production conditions in the optical sheet production process, in order to control the haze value (see PTLs 1 to 3).
However, contrast can differ even with the same haze value, and even with production using the haze value and the ratio of the internal haze and total haze as indexes, for example, it is not always possible to stably obtain a satisfactory antiglare sheet for an image display device.
In addition, it has been attempted to lower the reflectance by additionally providing a low-refraction light interference layer on the antiglare layer, but this requires precise formation of a film of about 100 nm, and is extremely expensive.
Furthermore, in recent years a variety of different kinds of viewing environments have appeared due to the diffusion and wider evolution of various types of delivery systems including one-seg, and the performance required for antiglare sheets has become ever more wide-ranging and personalized.
For example, with increasing opportunities for movie appreciation and the like, there is increasing demand for reproduction of dynamic images with high image quality in dark rooms, in order to experience a high-level viewing environment equivalent to that of a movie theatre, while as mobile usage continues to increase, there is also demand for image quality with physical strength and satisfactory balance between dynamic images and still images in light rooms, in order to bring out still images and dynamic images in the bright outdoors.
In other words, the image quality required for display terminals varies, and it is desired to develop an antiglare sheet for an image display device having performance suitable for the viewing environment.
PTL 4 and 5 indicate examples where the requirements differ depending on the viewing environment, and teach that still images and dynamic images have different requirements for performance, as well as different viewing conditions by observers.
As a result of diligent research on the problems described above, the present inventors have found that the sum of the internal diffusion and surface diffusion alone that has been considered in the prior art does not account for the total haze, but that in addition to the internal diffusion and surface diffusion, the total haze is also affected by the positional relationship between the diffusion particles and the surface irregularities.
The present inventors have also found, as a result of conducting diligent research on the performance required by antiglare sheets for image display devices suited both for high blackness in dark rooms and in light rooms and for high-level dynamic images and still images, such as for liquid crystal display devices (hereunder these will also be referred to simply as “for liquid crystal display devices”), and that in order to obtain a high level of blackness in dark rooms it is necessary to exhibit diffusion properties of a nature such that virtually no “stray light component” is produced, a factor that has not been considered in the past. The term “stray light component” refers to any of the uncontrollable light components traveling in directions inside the antiglare sheet different from the intended direction, among light impinging into the interior of the antiglare sheet, due to diffusion factors present on the surface of and/or inside the antiglare sheet (for example, on the surface they are the irregular shapes themselves, or in the interior, they are diffusion factors such as the particles used to form the irregular shapes), and they are usually reflected repeatedly inside the antiglare sheet.
It was also found that in order to obtain satisfactory image quality for viewing, it is important to sufficiently provide the regular reflection component of external light, which in the past has only been an object of prevention, while also considering the stray light components of projected light for dynamic images and still images in a light room.
In other words, in regard to the stray light components, when dark sections (for example, black) and light sections (for example, white) are present in the same screen, projected light in the light sections partially presents as stray light due to diffusion factors in the optical sheet, not only producing “flares”, or light emitted from dark sections, and lowering contrast, and especially reducing dark room contrast, but also causing loss of stereoscopic quality and resulting in images with poor planar variation.
The stray light component has minimal influence when viewing from the front, and tends to have a stronger influence when viewing from oblique directions.
In regard to the regular reflection component of external light, it was found that an optical film with extremely low regular reflection causes images to be perceived as simulated images, being subject to human sensory characteristics, whereas an optical film with an appropriate regular reflection component presents clear images and tends to result in their perception as actual objects, increasing the unique gloss and luminance of images on a dynamic image screen, to produce images with a sense of motion.
Such performance that includes contrast, a stereoscopic visual effect and sense of motion, that are required for such dynamic images (for example, for a scene with a youth under a blue sky, the black hair displayed on the screen is smooth black, the black pupils are moist black, and the skin is visible with the vivid brilliance characteristic of youth) will be referred to as “vivid complexion and blackness”.
In addition, for film viewing under illumination, or for mobile purposes, a property of preventing unwanted reflection (an antiglare property) is desired even for viewing of dynamic images. An “antiglare property for dynamic images” is a property that is not completely free of virtual images of objects in front of the image display device, but rather slightly prevents unwanted reflection, where the outlines and borderlines of objects on borders and backgrounds are slightly halated for an observer observing the dynamic images.
Moreover, in recent years, there is a demand for antiglare sheets for liquid crystal display devices with excellent “blackness in dark surroundings”, which is a degree of notable, high-level blackness under high-level viewing conditions, such as for film appreciation, or in other words, viewing under dark room conditions without external light, and in the optimal range of the display device (a viewing range that allows viewing with a front luminance of 33.3% or greater).
Still images must have excellent contrast and greater prevention of unwanted reflection, and such performance of contrast and prevention of unwanted reflection required for still images will be referred to as “image crispness”.
In other words, there is an increased preference that antiglare sheets for liquid crystal display devices should have excellent vivid complexion and blackness and image crispness.
Evaluation of image quality has included the “black tightness” mentioned in PTL 6 and “glazed black feel”, mentioned in PTL 7.
In order to improve narrowness of angle, which is a fundamental defect in liquid crystal displays, antiglare sheets are often provided with diffusibility. However, providing diffusibility can lower contrast, especially for frontal viewing.
Black tightness is evaluated as a compromise between viewing angle enlargement and contrast, and by comparing blackness during power-off and blackness during power-on (black images) when viewing directly from the front of the display, with a more intense blackness being evaluated as a more powerful tight feel for the screen.
In addition to stray light components that are very weak in the front and more noticeable at greater oblique angles, in a liquid crystal display system structure the light leaking from the liquid crystal display unit itself (leaked light) is present even during black display, and therefore the blackness during power-on, as seen directly from the front, is the level of blackness resulting from a combination of this leaked light and external light reflection, while blackness during power-off is the blackness with only external light reflection, since no projected light is present.
Stated differently, “black tightness” means an intense level of blackness against both external light and leaked light, without considering the stray light components, unlike the aforementioned vivid complexion and blackness, and also without consideration of an appropriate necessary level for the regular reflection component, and therefore even if the contrast is high, the gloss and luminance of the image is inferior, no sense of motion is produced, and the vivid complexion and blackness is not increased. In particular, since increasing diffusion and widening the viewing angle are a priority, stray light components are easily produced and the blackness in dark surroundings tends to be reduced.
Furthermore, a “glazed black feel” is black reproducibility when an image display device displays black in a light room environment, i.e., abundant expression of graded black, by minimizing diffusion of the non-regular reflected light component of light incident to the optical laminate from the exterior, preventing non-regular reflected light from reaching the eye of the observer, and visual evaluation is made under three band fluorescence, after attachment to a cross nicol polarizing plate or a black acrylic board via an acrylic pressure-sensitive adhesive for optical films (product with total light transmittance: ≧90%, haze: ≦0.5%, film thickness: 10 to 55 μm, such as the MHM Series by Nichiei Kakoh Co., Ltd., or trade name: “L8010” by Hitachi Chemical Co., Ltd.) on the side opposite the film side of an optical laminate.
That is, with this measurement method, evaluation of dynamic images is not performed and the effect of stray projected light components is completely ignored. Therefore, even with high gloss and luminance, no dark room contrast or stereoscopic visual effect is produced, and the vivid complexion and blackness is not increased.
“Contrast” is the ratio of white luminance to black luminance, and since the absolute value of black luminance is much smaller than the white luminance, the effect of black luminance on contrast is greater. In order to obtain images with excellent contrast, it is necessary to have excellent “black tightness”, as light room blackness for a wide viewing angle, “blackness in dark surroundings” as the absolute blackness, and “glazed black feel” as abundant graded expression in the black region.
Also, in order to present both still images and dynamic images, it is necessary to exhibit excellence at least in terms of vivid complexion and blackness with a stereoscopic visual effect and sense of motion.
In PTLs 8 and 9, which limit the diffusion property of antiglare sheets, the contrast is satisfactory, but no consideration is given to the issues of physical performance including adhesiveness and hard coat properties, which are indispensable for practical use, or glare and presentation of both dynamic images and still images, and therefore sufficient performance has not been exhibited.